MBS and CRT Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Information about MBS and CRT securities
Minimum term of fixed rate mortgages underlying MBS	15 years
Principal/ Current Face	$ 10,172,893,000	$ 11,703,896,000
Purchase Premiums	138,551,000	172,160,000
Accretable Purchase Discounts	(283,799,000)	(317,930,000)
Discount Designated as Credit Reserve and OTTI	(694,241,000)	(787,541,000)
Amortized Cost	9,336,008,000	10,771,616,000
Fair Value	9,969,163,000	11,356,643,000
Gross Unrealized Gains	669,553,000	656,956,000
Gross Unrealized Losses	(36,398,000)	(71,929,000)
Net Unrealized Gain/(Loss)	633,155,000	585,027,000
Principal payments receivable	2,600,000	1,000,000
Agency MBS
Information about MBS and CRT securities
Principal/ Current Face	3,581,302,000	4,550,403,000
Purchase Premiums	135,182,000	172,087,000
Accretable Purchase Discounts	(51,000)	(59,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	3,719,037,000	4,723,462,000
Fair Value	3,738,497,000	4,752,244,000
Gross Unrealized Gains	50,653,000	69,207,000
Gross Unrealized Losses	(31,193,000)	(40,425,000)
Net Unrealized Gain/(Loss)	19,460,000	28,782,000
Agency MBS | Fannie Mae
Information about MBS and CRT securities
Principal/ Current Face	2,879,807,000	3,690,020,000
Purchase Premiums	108,310,000	139,243,000
Accretable Purchase Discounts	(51,000)	(59,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	2,988,066,000	3,829,204,000
Fair Value	3,014,464,000	3,865,485,000
Gross Unrealized Gains	45,706,000	62,111,000
Gross Unrealized Losses	(19,308,000)	(25,830,000)
Net Unrealized Gain/(Loss)	26,398,000	36,281,000
Agency MBS | Freddie Mac
Information about MBS and CRT securities
Principal/ Current Face	693,945,000	851,087,000
Purchase Premiums	26,736,000	32,680,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	723,285,000	884,798,000
Fair Value	716,209,000	877,109,000
Gross Unrealized Gains	4,809,000	6,906,000
Gross Unrealized Losses	(11,885,000)	(14,595,000)
Net Unrealized Gain/(Loss)	(7,076,000)	(7,689,000)
Agency MBS | Ginnie Mae
Information about MBS and CRT securities
Principal/ Current Face	7,550,000	9,296,000
Purchase Premiums	136,000	164,000
Accretable Purchase Discounts	0	0
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	7,686,000	9,460,000
Fair Value	7,824,000	9,650,000
Gross Unrealized Gains	138,000	190,000
Gross Unrealized Losses	0	0
Net Unrealized Gain/(Loss)	138,000	190,000
Non-Agency MBS
Information about MBS and CRT securities
Principal/ Current Face	6,206,598,000	6,961,493,000
Purchase Premiums	57,000	73,000
Accretable Purchase Discounts	(278,191,000)	(312,182,000)
Discount Designated as Credit Reserve and OTTI	(694,241,000)	(787,541,000)
Amortized Cost	5,234,223,000	5,861,843,000
Fair Value	5,825,816,000	6,420,817,000
Gross Unrealized Gains	596,795,000	587,331,000
Gross Unrealized Losses	(5,202,000)	(28,357,000)
Net Unrealized Gain/(Loss)	591,593,000	558,974,000
Credit Reserve	675,600,000	766,000,000
OTTI	18,600,000	21,500,000
Non-Agency MBS | Expected to Recover Par
Information about MBS and CRT securities
Principal/ Current Face	2,847,398,000	2,906,878,000
Purchase Premiums	57,000	73,000
Accretable Purchase Discounts	(24,273,000)	(31,576,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	2,823,182,000	2,875,375,000
Fair Value	2,847,291,000	2,878,532,000
Gross Unrealized Gains	26,477,000	23,300,000
Gross Unrealized Losses	(2,368,000)	(20,143,000)
Net Unrealized Gain/(Loss)	24,109,000	3,157,000
Non-Agency MBS | Expected to Recover Less Than Par
Information about MBS and CRT securities
Principal/ Current Face	3,359,200,000	4,054,615,000
Purchase Premiums	0	0
Accretable Purchase Discounts	(253,918,000)	(280,606,000)
Discount Designated as Credit Reserve and OTTI	(694,241,000)	(787,541,000)
Amortized Cost	2,411,041,000	2,986,468,000
Fair Value	2,978,525,000	3,542,285,000
Gross Unrealized Gains	570,318,000	564,031,000
Gross Unrealized Losses	(2,834,000)	(8,214,000)
Net Unrealized Gain/(Loss)	$ 567,484,000	$ 555,817,000
Current face amount of non-Agency MBS expected to be recovered (percent)	89.00%	89.00%
Total MBS
Information about MBS and CRT securities
Principal/ Current Face	$ 9,787,900,000	$ 11,511,896,000
Purchase Premiums	135,239,000	172,160,000
Accretable Purchase Discounts	(278,242,000)	(312,241,000)
Discount Designated as Credit Reserve and OTTI	(694,241,000)	(787,541,000)
Amortized Cost	8,953,260,000	10,585,305,000
Fair Value	9,564,313,000	11,173,061,000
Gross Unrealized Gains	647,448,000	656,538,000
Gross Unrealized Losses	(36,395,000)	(68,782,000)
Net Unrealized Gain/(Loss)	611,053,000	587,756,000
CRT securities
Information about MBS and CRT securities
Principal/ Current Face	384,993,000	192,000,000
Purchase Premiums	3,312,000	0
Accretable Purchase Discounts	(5,557,000)	(5,689,000)
Discount Designated as Credit Reserve and OTTI	0	0
Amortized Cost	382,748,000	186,311,000
Fair Value	404,850,000	183,582,000
Gross Unrealized Gains	22,105,000	418,000
Gross Unrealized Losses	(3,000)	(3,147,000)
Net Unrealized Gain/(Loss)	$ 22,102,000	(2,729,000)
3 Year Step-up securities
Information about MBS and CRT securities
Basis points for coupon step-up	300.00%
Coupon step-up period	36 months
Principal/ Current Face	$ 2,700,000,000	2,600,000,000
Amortized Cost	2,700,000,000	2,600,000,000
Fair Value	2,700,000,000	2,600,000,000
Debt Securities | CRT securities
Information about MBS and CRT securities
Fair Value	271,200,000	62,200,000
Gross Unrealized Gains	12,700,000	332,000
Gross Unrealized Losses	(3,000)	(555,000)
Net Unrealized Gain/(Loss)	$ (3,000)	$ (223,000)